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SpaceX is excited to begin counting down tonight to a 4:55 a.m. Saturday launch from Cape Canaveral, embracing the demonstration mission's potential history-making significance while working to manage expectations.

"We know this has been touted as a huge mission -- we keep trying to say it's a test," company President Gwynne Shotwell said during a pre-launch press conference this afternoon at Kennedy Space Center. "Success is not going to be the success of the commercial space industry, and failure is not going to mean failure of the commercial space industry."

SpaceX's Falcon 9 rocket will attempt to lift an unmanned Dragon capsule from Launch Complex 40 on its way to the International Space Station where, after several days of tests, it could become the first commercial spacecraft to dock at the outpost.

The weather forecast remains favorable, with a 70 percent chance of conditions good enough to permit a launch. A recent spate of thunderstorms isn't expected to present a problem, though there's a chance of thick clouds.

"Since this is an early morning launch, that is typically -- during the summertime, anyway -- our best weather time of the day," Joel Tumbiolo, the launch weather officer from the Air Force's 45th Weather Squadron.

The demonstration mission is the second under a NASA program developing new vehicles for commercial deliveries of cargo to the station.

Officials said today the mission is far more complex than the first demonstration in December 2010, which launched a Dragon in to space for two orbits of Earth and recovered it after reentry.

The Dragon now set for launch must deploy solar arrays to generate power, employ a new cooling system and test sensors and communications systems needed to approach and berth with the station.

"This is hard. This mission is extremely complicated," said Alan Lindenmoyer, NASA manager of the demonstration program. "The energy levels involved, the precision that is required to get thousands of parts to all work perfectly together to achieve orbit and berth it with the space station is extremely complex."

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As a result, he said, the mission would be successful if it achieves any of its goals, and unfinished goals could be rolled into a later flight. The mission's goals combines what were once planned as two separate missions.

Any problem that arises during the flight, officials said, would present a learning opportunity.

If all goes well, about 75 hours would elapse between launch and a berthing at the station, when a robotic arm would grab the Dragon and pull it into a docking port.

"I think we're going to be biting off our fingers between now and Hour 75," said Shotwell, asked about which moments would be the biggest nail-biters. "I don't think there's going to be a lot of sleep for the next 70 or 75 hours for the folks at SpaceX. It's all hard."

The mission was delayed from late April to give SpaceX more time to test and for NASA to confirm that the software systems that will guide the automated Dragon vehicle are fully ready to go. Shotwell said she knew of "no issues" remaining with the software.

While not promising success, Shotwell acknowledged that a Dragon berthing at the station would represent a major achievement for commercial space industry.

So far, only nationally designed and operated vehicles from the United States, Russia, Japan and Europe have visited the station.

"If successful, there's no question this is a historic flight," Shotwell said. "We really stand in awe of having the opportunity to attempt this."

If the mission is a success, it would mark the end of SpaceX's participation in the demonstration program that so far has invested $381 million out of a potential $396 million in the vehicles.

Then, later this year, SpaceX would begin executing the first of a dozen station resupply missions under a $1.6 billion NASA contract.

Each launch attempt has a near-instantaneous window. If there's a scrub Saturday, the next available attempt would be Tuesday.

After Saturday, the best potential launch dates based on propellant usage are May 19, May 22, May 25 and May 29, with less favorable options May 23 and May 26.

After May 29, the sun's angle against the space station would create hot temperatures that would push a launch to mid-June.

The launch will be the third by the Falcon 9 rocket, which blasted off in June and December of 2010. It would be the Dragon's second flight.

Weather aside, SpaceX has yet to complete a Falcon 9 countdown without interruption, so simply getting off the ground within the tight launch window presents a significant challenge.